The present invention relates to a method and apparatus for locating faults in a network of signal carriers such as electrical or optical cabling systems. In particular, it relates to networks which contain multiple joints, such as low voltage electrical signal or power distributions systems.
Locating faults in underground cabling systems is currently achieved using a pulse echo technique. In the pulse echo technique, an interrogation pulse is transmitted along the system from a test site, this pulse is reflected by any fault in the cable system, and the elapsed time between the initial pulse and the reflected pulse being received at the test site is used to calculate the distance of the fault from the test site.
This technique works well in systems which do not have nodes (such as joints) which generate reflections of the pulse even though they are not faulty. However, when this technique is used in systems with multiple joints (such as low voltage electrical distribution systems or telecommunications systems) then each joint may introduce a reflection of the pulse in addition to the reflection generated by the fault or disruption.
Thus, data produced by the pulse echo technique performed on a system with multiple joints is difficult to interpret. Skilled interpretation of this data is required to distinguish between reflections from joints and reflections from faults.
Even with skilled interpretation of the data from a multijoined system where there may be more than one possible location of the fault, it is difficult to locate the fault to an accuracy of less than several meters for each of these possible locations.
Using standard echo pulse techniques for tracing faults in multijoined systems (particularly where the cable is buried underground or is otherwise inaccessible) is very expensive because of the uncertainty in distinguishing between reflections due to joints and reflections due to faults, and because of the inaccuracy in determining the precise location of the faults.
It is an object of the present invention to improve fault-finding in cabling systems. It is another object to distinguish between faultless nodes (which produce reflections) and faults. It is a further object to reduce the amount of skill needed to interpret the results of fault location apparatus. It is a further object to automate the fault locating process.
The general solution to the above problem is to compare pairs of signals from different signal carriers of the network under test and adaptively process the signals to determine any key deviations. These deviations may indicate the locations of the faults.
According to a first aspect of the present invention there is provided a method for locating faults in a network of signal carriers comprising establishing a test site where signal carriers of the network can be accessed: delivering an interrogation pulse-like waveform along a first set of signal carriers; receiving a first composite reflected signal from the first set of signal carriers and which is a time-distributed collection of individual reflections; sampling the first composite reflected signal at time intervals and storing the samples; delivering the same interrogation pulse-like waveform along a second set of signal carriers; receiving a second composite reflected signal from the second set of signal carriers and which is a time-distributed collection of individual reflections; sampling the second composite reflected signal at time intervals and storing the samples; consecutively comparing in pairs, samples of the first composite reflected signal with those of the second composite reflected signal using the function of an adaptive filter and comparator arranged consecutively to adapt the samples of the first composite reflected signal to minimise the different signal, between the output of the adaptive filter and the second composite reflected signal, at the output of the comparator; identifying the time interval at which the difference signal exceeds a threshold value; and scaling the time domain position of the identified time interval to represent the distance to a fault in the signal carriers from the test site.
According to a second aspect of the present invention there is provided a fault locator for carrying out the method of the first aspect comprising an adaptive filter and a comparator.
By virtue of the present invention the location of the fault position is automatically obtained from the filtered error signal due to the adaptive processing effectively learning about the similarities between cable characteristics for all locations prior to the fault and utilising this knowledge to determine where significant departure from the norm exists.
It will be understood that the pulse-like waveform is a finite-duration waveform and for example may be a simple pulse of square, rectangular or triangular shape or it may be of damped sinusoid form for example having only two sinusoidal periods of which the amplitude of the second is less than that of the first.
The method and apparatus may be operated and controlled remotely from the test site and may be operated in a trigger mode where the trigger signal is provided automatically resulting from an event of interest such as a voltage or current surge.
Furthermore the network of signal carriers may be arranged to carry electrical signals and/or power in which case the interrogation waveform is itself an electrical signal or the network may be arranged to carry optical signals in which case the interrogation waveform is itself an optical signal for example a transformed electrical signal, and of course the composite reflected signals are transformed to electrical signals for processing.
The network may have any number of signal carriers per cable, for example, ten or more as is conventionally used in telecommunications, or it may have only three carriers per cable as is conventionally used in electrical power distribution networks.